Scientists at The Babraham Institute in Cambridge found that green vegetables are the source of a chemical signal that is important to the functioning of the immune system. Green vegetables ensure that intra-epithelial lymphocytes (IELs), a specialized type of white blood cell located in the gut and in the skin, function properly.

'[S]uch a clear cut interaction with the diet was unexpected.'

Researchers fed healthy mice a purified diet with almost completely no vegetables for two to three weeks. During that time 70 percent of the IELs lining the intestinal tract disappeared. Consequently, the mice were more susceptible to infection and had a more fragile intestinal lining which increased their risk of inflammation - a surprise to researchers.

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“This was surprising, since the new diet contained all other known essential ingredients such as minerals and vitamins,” said Marc Veldhoen, senior author of the paper. "I would have expected cells at the surface would play some role in the interaction with the outside world, but such a clear cut interaction with the diet was unexpected. "

IELs comprise a defensive network under the layer of epithelial cells covering both inner and outer body surfaces. They are important for protecting against infection, maintaining a healthy gut, and healing wounds. IELs also help to maintain a healthy balance of "good" and "bad" bacteria and destroy infected cells that may be harmful.

According to Veldhoen and his team of researchers, the number of IELs depend on levels of aryl hydrocarbon receptor (AhR), a cell-surface protein which is regulated by a chemical component in cruciferous vegetables – those from the mustard or cabbage families – such as cabbage, broccoli, and Brussels sprouts. In the case of the mice, the loss of the AhR receptor caused a loss of microbes that live on the intestinal surface as well as a change in the composition of the microbes causing the mice to become susceptible to an artificially-induced form of an inflammatory bowel condition.

Dr. Brigitta Stockinger, Head of the Division of Molecular Immunology at the National Institute for Medical Research stated, “The food we eat plays a crucial role in influencing our immune system and we have been looking at the intricate biology that determines how cells in our intestines maintain an intrinsic protection against microbes. This study in mice is an important step towards increasing our understanding of how environmental signals shape immune responses at barrier sites such as the intestine."

Some of the characteristics observed in the mice are consistent with certain clinical observations observed in patients with inflammatory bowel disease; however, it is important to note that the results of this study cannot be extrapolated to humans because there may be other factors at work that have not yet been discovered.

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Nevertheless, nutrition experts agree that most adults need to eat at least three cups of vegetables a day. The 2010 Dietary Guidelines for Americans recommend eating a variety of vegetables with an emphasis on dark-green vegetables, as well as red and orange vegetables.